U.S. Pat. No. 4,540,738 issued to Zimmermann, Sept. 10, 1985 and assigned to The Kendall Company, assignee of the present invention, relates to improving the stability of acrylic adhesive compositions comprising a free radical polymerizable acrylate or methacrylate monomer and an initiator. In general, the adhesive compositions to which the patented invention are directed may be described as "two part adhesive compositions" which when admixed with an activator polymerize to provide a solid adhesive possessing excellent physical properties.
As stated therein, these polymerizable adhesive compositions tend to gel during storage, thereby materially limiting their shelf life. In general, this tendency to gel or polymerize in storage increases as the temperature increases. Conversely, lower temperature storage of the adhesive material results in an extension of the shelf life. Accordingly, refrigeration prior to use is often required. However, this technique to increase adhesive shelf life is not only cumbersome and expensive, but also requires rewarming prior to use to attain a reasonable cure rate.
To obviate this problem, procedures are known in the prior art for the addition of various stabilizers in lieu of refrigeration. Ideally, these additives should impart the desired shelf life without adversely affecting the adhesive properties. However, many of these additives tend to lower the cure speed and/or the adhesive strength.
According to the patented invention, a stabilizing system is incorporated in the acrylic composition comprising effective amounts of a polymerization inhibitor, specifically hydroquinone, and a metal chelating agent, specifically an alpha amino phosphonic acid or alkali metal salt thereof.
However, quite surprisingly, for reasons which are not clearly understood, the invention described in U.S. Pat. No. 4,540,738 has been found not to be applicable for reproducible large scale production, thereby precluding commercialization of the adhesive compositions. Specifically, the patented stabilizing system has been found not to consistently prevent gelation during storage at room temperature or higher.
The present invention is directed to a solution to the problem in manufacture of the stabilizing system of U.S. Pat. No. 4,540,738 and, in this context, may be said to be an improvement over the invention described and claimed therein.